Flue gas from combustion sources, for instance motors, often contains pollutive matter. Such contaminants may be removed in a cleaning process. Such a cleaning process may be a scrubber. Scrubbers for cleaning flue gases may broadly be divided into two types, so called wet scrubber and dry scrubber. The main purpose for the scrubber is to remove or neutralize sulphur components from the flue gas, especially sulphur oxide. In addition one may often see that, for a wet scrubber, this will remove particulate matter as well, such as heavy metal, PAH (poly aromatic hydrocarbons), soot and sulphur bonded particles which may be pollutive as atmospheric emission. For marine use usually fuel oil, semi refined heavy oil, is used as fuel. Such oil may have a sulphur content of 4-5%. The limit for allowed sulphur in the exhaust is constantly being lowered. From 2015 within certain sea areas the exhaust should not contain more than maximum about 22 ppm sulphur or a level corresponding to a level equivalent to the use of fuel containing 0.1% sulphur.
A wet scrubber uses a liquid, a so called scrubber liquid, which is sprayed into the gas flow, preferable sprayed out as a mist, in a way that the gas will pass a thin layer of such a scrubber liquid. Depending of use, a scrubber liquid may be fresh water, sea water, slurry, or for instance gray water onboard ships. Scrubbers used for cleaning exhaust gases from larger combustion engines onboard ships often uses seawater or fresh water as a scrubber liquid.
To remove sulphur from the gas the main principle consists in letting the gas react with water to form sulphuric acid. Sea water has a natural buffering capacity for this amount of sulphuric acid. When fresh water in a closed, re circulating system is used, it is, in lack of natural buffering capacity, added a neutralizing chemical, e.g. sodium hydroxide.
It is important for the purification efficiency that the contact between the scrubber liquid and the gas to be cleaned, are as good as possible. Different designs for scrubbers are therefore available.
U.S. Pat. No. 7,976,809B2 describes a scrubber for reduction of sulphur in the exhaust from a marine engine. The Scrubber utilizes fresh water with an additive, NaOH, as a scrubber liquid. The scrubber is a vertical extending tank with one or two liquid spray arrangements with nozzles arranged above the appurtenant packed bed wherein the liquid meets the gas inside and around the packed bed. The exhaust which is wetted by the scrubber liquid passes a demister before the gas is discharged.
WO9944722 describes a scrubber which combines a venturi scrubber and a vertical extended “packed-bed” scrubber. The venturi part is incorporated in the scrubber tank or is placed outside the tank in a slightly inclined design. Typical to this is that the gas and the liquid in the venturi part is flowing downwards.
US20100206171A1 describes a desulphurization equipment for exhaust gas from a marine engine with the use of seawater as scrubber liquid. Also in this scrubber of the packed bed type where the scrubber liquid is added above a so called packed bed and the gas is guided counter current relative to the water flow through the packed bed scrubbing step. The scrubber liquid is removed in the bottom of the scrubber. The lower part is used as a cooling step as well. After the scrubber the used scrubbing liquid is directed for the particles to a purifying step and a neutralization step for the formed acid. The gas inlet is in the bottom step either laterally horizontal or vertically through a feeding pipe with a hat to prevent the downwards flowing liquid to flow into the gas inlet pipe.
WO2012113977A1 relates to a scrubber system for treating exhaust gas in a marine vessel with a water based solution. The scrubber system comprising a first scrubber unit and a second scrubber unit. The exhaust gas inlet located in the first scrubber unit and with the exhaust gas outlet in the second scrubber unit and a second conduit section connecting the first scrubber unit to the second scrubber unit. Each scrubbing unit having separate scrubbing medium circuits. The first scrubber medium circuit is provided with a source of scrubbing solution comprising a first connection to outside of the hull beneath the water line of the vessel and that the second scrubber medium circuit is provided with a source of scrubbing solution comprising a source of fresh water in the vessel.
The above mentioned scrubber designs will have different cleaning efficiencies depending of the velocity and the pressure of the gas and the pressure drop through the scrubber. Externally arranged fans or pumps may be used to force the exhaust through the scrubber. There is often a need for large volumes of water and several steps of wetting the gas. Such designs often becomes space demanding.
The need of less space demanding scrubbers with good efficiency is all over growing. The requirement for purification of the discharged exhaust is continuously tightened up. Onshore the space is necessarily not as important but if the scrubber will be used for instance onboard a ship the available space for the scrubber and the scrubbing arrangement will be limited. Especially the available space will be limited if a scrubber is to be mounted to satisfy the severe limits for pollution from the exhaust gas onboard an existing ship. Onboard a passenger ship the space will already be utilized to the maximum and available space for later installation will be limited and costly. Another aspect to wish for a small and compact installation may be regarding to stability; that one do not wish to place large heavy pipe arrangement high up in the ship.
A limitation for size reduction is that when the scrubber size is reduced, the flue gas speed will rise and the pressure drop over a standard packed scrubber will reach a maximum limit.